1. Field of the Invention
This invention relates to a multiplex stereo demodulator for use in an FM radio receiver.
2. Prior Art
Conventionally, there have been proposed two types of multiplex stereo demodulators for use in FM radio receivers, one of which is an analog multiplication type demodulator and the other of which is a switching type demodulator.
The analog multiplication type demodulator comprises, as shown in FIG. 1, a subcarrier generating circuit 10 which generates sinusoidal subcarrier signals S.sub.1 and S.sub.1 both of 38 KHz in accordance with a pilot signal contained in a composite signal applied to an input terminal 11. These sinusoidal subcarrier signals S.sub.1 and S.sub.1 are supplied respectively to analog multipliers 12 and 13 to be multiplied by the composite signal, whereby left channel and right channel audio signals are obtained at output terminals L.sub.CH and R.sub.CH, respectively. This type of demodulator is disadvantageous in that it is significantly difficult to obtain sinusoidal subcarrier signals precisely synchronized with each other, so that the channel separation is liable to be deteriorated due to the occurrence of the phase shift (non-synchronization) of the sinusoidal subcarrier signals.
The switching type demodulator comprises, as shown in FIG. 2, a subcarrier generating circuit 20 which generates square wave subcarrier signals .phi. and .phi. both of 38 KHz in accordance with a pilot signal contained in a composite signal applied to an input terminal 21. These square wave subcarrier signals .phi. and .phi. are supplied respectively to transistors Qa and Qb to alternately conduct them, whereby a left channel audio signal and a right channel audio signal are obtained at output terminals L and R, respectively. This type of stereo demodulator is simpler in construction than the aforesaid analog multiplication type demodulator but is disadvantageous in the following respects: (1) Circuits disposed on the periphery of the demodulator are subjected to radiation of higher harmonics of the subcarrier signals which is due to the switching operation in the demodulator performed in accordance with the square wave subcarrier signals; (2) Noises contained in the composite signal and having frequencies corresponding to the higher harmonics of the 38-KHz subcarrier signals (i.e., 114 KHz, 190 KHz and so on) can be demodulated by the switching operation performed in accordance with the 38-KHz square wave subcarrier signals. The demodulated higher harmonics cause an interference due to their beats; and (3) High-speed circuit elements for effecting the switching operation are needed.